For an on-board fuel cell system for automobiles and the like, various hybrid fuel cell systems provided with both a fuel cell and a battery as a power source have been proposed for addressing situations such as rapid load variation beyond the power generating capacity of a fuel cell.
In a hybrid fuel cell system, the output voltage and others of the fuel cell are controlled by a DC-DC converter. A widely used DC-DC converter for such control is of a type that operates switching elements, such as power transistors, IGBTs, or FETs, in a PWM fashion to convert a voltage. With the advance of electronic equipment of smaller size with less power consumption and higher performance, there is a demand for a DC-DC converter with a faster speed, a larger capacity, and reduced ripples. In order to meet such a demand, a multiphase DC-DC converter is conventionally used, which is made up of multiple DC-DC converters connected in parallel to each other (see Patent Literature 1).